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Understanding Your Melanoma Pathology Report, Part I: Mitotic Rate

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Updated June 09, 2014

Mitosis during cell division is shown on a the melanoma pathology report

Measuring the number of dividing cells can help predict survival in patients with melanoma

photo © NY Department of Health
One way to better understand your melanoma diagnosis and the resulting treatment strategy is to read your melanoma pathology report, which is sent to your doctor and contains critical information such as the exact stage of your disease. This is the first part of a series of articles that will help you decipher the technical jargon in these reports.

Diagnosis Overview

If a suspicious lesion or mole is found during your skin exam, your primary care physician or dermatologist will take a biopsy specimen for the pathologist (a physician who examines tissues and fluids to diagnose disease in order to assist in making treatment decisions) to examine under a microscope.

If the pathologist finds malignant (cancerous) cells in the biopsy, your primary care physician may order other tests -- lymph node, blood, urine and imaging tests -- to find out whether or not the cancer has spread. These tests help the pathologist assess the location, spread and stage of the melanoma. The pathologist consults with your primary care physician after reviewing the test results and determining the stage of the cancer. Together, they determine treatment options most appropriate for your condition.

Mitotic Rate

Your pathology report contains information, such as tumor stage, Clark level, Breslow thickness, ulceration (occurs when melanoma breaks through the overlying skin) and mitotic rate (MR). A high mitotic rate also correlates with a greater likelihood of having a positive sentinel lymph node biopsy.

The MR is measured by simply examining the excised (surgically removed) tumor with a microscope and manually counting the number of cells exhibiting mitosis, an easily identifiable characteristic of dividing cells. Most often, the MR is reported as one of three categories (although it is sometimes listed as a continuous, uncategorized number):

  • less than 1 per square millimeter
  • 1 to 4 per square millimeter
  • greater than 4 per square millimeter

The higher the mitotic count, the more likely the tumor is to have metastasized (spread). The logic is that the more cells are dividing, the more likely they will invade the blood or lymphatic vessels and thus spread around the body.

Research has shown that the odds of survival for patients with stage I melanoma and a mitotic rate of 0 per square millimeter is twelve times that of patients with a mitotic rate of greater than 6 per square millimeter. Also, only 4% of lesions with low MR recur (come back) compared to 24% of those with a high MR. Mitotic rate can also help predict if your sentinel lymph node biopsy will be positive or not.

Is Measuring MR Worthwhile?

Since the 1990s, many studies have confirmed that the mitotic rate is a significant predictor of outcomes in patients with melanoma, although some controversy still exists. Two issues are under debate: 1) is MR independent of other prognostic factors? and 2) if not, is measuring MR worth the time and expense?

Although MR has no role in the current staging system for melanoma, research has demonstrated that it is a more important prognostic factor than ulceration, which does have an important role in staging. Some doctors, however, believe that the mitotic rate is not an independent prognostic factor, because it is closely related to tumor (Breslow) thickness and ulceration. For example, the American Academy of Dermatology argues that MR should be optional in biopsy reports. On the other hand, the National Comprehensive Cancer Center recommends that MR should be reported for all lesions in stage I to II patients. Still other experts argue that measuring the MR should only be done in large academic (university) medical centers for future research purposes. If the MR isn't included in your pathology report, be sure to ask your doctor about his or her reasoning.

Conclusion

Always request a copy of your pathology report. Read it and ask your physician questions about it. Don't hesitate to get a second opinion about the diagnosis from a specialist, such as a dermatopathologist. A knowledgeable patient is an empowered patient, and an empowered patient can make better treatment choices that lead to better outcomes.

More on How to Interpret Your Melanoma Pathology Report

Sources:

Melanoma. National Comprehensive Cancer Network. V1.2009. 2 December 2008.

Understanding a Cancer Diagnosis: Skin Melanoma. College of American Pathologists. 2 December 2008.

Attis MG, Vollmer RT. "Mitotic rate in melanoma: a reexamination." Am J Clin Pathol 2007 127(3), 380-4. 2 December 2008.

Barnhill RL, Katzen J, Spatz A, Fine J, Berwick M. "The importance of mitotic rate as a prognostic factor for localized cutaneous melanoma." J Cutan Pathol 2005 32 268-273. 2 December 2008.

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